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This work presents the first comparative study on syntheses, characterization, and applications of different clay/gold nanoparticle composites (clay/AuNPs) as acetylcholinesterase (AChE) carriers and modified-electrode materials to detect chlorpyrifos, an organophosphate pesticide. The selected based-clays were plate-like kaolinite (Kaol; 1:1 aluminum phyllosilicate), globular montmorillonite (Mt; 2:1 aluminum phyllosilicate), globular bentonite (Bent; 2:1 aluminum phyllosilicate), and fibrous sepiolite (Sep; 2:1 inverted ribbons of magnesium phyllosilicate). The clay/AuNPs were synthesized using in situ chemical reduction of the gold precursor with anchoring 3-aminopropyl triethoxysilane (APTES) on the clay surfaces. The studied materials were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), nitrogen adsorption/desorption, Fourier transformed infrared (FTIR) spectroscopy, thermogravimetric analyzer (TGA), zeta-potential analysis, and electrochemical impedance spectroscopy (EIS). Surface area, surface charges, and hydrophilic/hydrophobic nature of based-clays and the clay/AuNPs played major roles on AChE loading, residual activity, and sensor storage stability. The Mt/AuNPs was determined, among the studied clays, as the best supports for AChE loading and residual activities due to its high specific surface area and weak electrostatic interaction with the immobilized enzyme. Since apparent catalytic efficiencies kcatapp/KMappof the immobilized AChE in the four clay/AuNPs were found statistically similar, Mt/AuNPs which contained the highest enzyme loading was the favorite modified-electrode material. For chlorpyrifos detection, Kaol/AuNPs-based sensors resulted in the lowest detection limit, presumably due to the strong hydrophobic surface that helps to concentrate the nonpolar pesticide to the vicinity of the immobilized enzyme. Unfortunately, the highest electrostatic repulsion between the Kaol/AuNPs and AChE, and the dehydrated Kaol surface resulted in the lowest storage stability of the AChE pesticide sensors. Overall, Mt/AuNPs showed the highest potential as the AChE carrier and the modified electrode material for chlorpyrifos determination.

Zinc oxide (ZnO) nanoparticles containing oxygen vacancies were synthesized by the French process modified with pulsed injection of nitrogen. Zinc vapor was generated by evaporation of zinc foil and carried by a carrier gas to react with co-currently supplied air. During the reaction, nitrogen gas was injected in pulse, perpendicular to the flow direction of both zinc vapor and air. Low partial pressure of oxygen and turbulence caused by pulsed injection yielded uniform ZnO nanotetrapods that contained oxygen vacancies. The content of oxygen vacancies depended upon the characteristics of the pulse, i.e. flowing and non-flowing period of the gas, pulsing cycle time, and the supplied pressure of the injected gas. Strong correlation between the presence of oxygen vacancies and the intensity of green emission in the photoluminescence spectra of ZnO was also observed.

Titanium dioxide nanoparticles were prepared using solvothermal methods in 1,4-butanediol and toluene. Three different crystallite sizes (10 nm, 13 nm, and 17 nm) of titania as anatase crystals were produced and were then employed as catalysts in photocatalytic oxidation of ethylene in gas phase. As the crystallite size of titania increased, more Ti3+ surface defects were observed. The amounts of surface defects on titania were determined by temperature-programmed desorption using carbon dioxide as a probe molecule. Despite the smaller specific surface area, titania with larger crystallite size exhibited higher photocatalytic activity, which was attributed to more surface defects in the catalyst.

Nano-TiO2 powders were synthesized by the solvothermal method under various reaction conditions in order to obtain average crystallite sizes of 9–15 nm. The amounts of surface defect of TiO2 were measured by means of temperature-programmed desorption of CO2 and electron spin resonance spectroscopy. It was found that the ratios of surface defect/specific surface area increased significantly with increasing TiO2 crystallite size. The TiO2 with higher amounts of surface defects exhibited much higher photocatalytic activity for ethylene decomposition.

Volatile organic compounds present in air at low concentrations can be oxidized to carbon dioxide and water when exposed to thin films of titania irradiated with near UV light. Borosilicate glass rings were coated with a titania-sol generated by controlled hydrolysis of titanium isopropoxide. A reactant stream containing either 48.99 or 1055 ppm of ethylene was fed continuously to a tubular reactor packed with the coated glass rings and surrounded by blacklight blue fluorescent bulbs. The effluent was analyzed by gas chromatography. Titania supported on glass rings exhibited higher photocatalytic activity than unsupported titania pellets. Preliminary kinetic studies of photocatalytic oxidation of ethylene suggest that the reaction rate can best be described using a Langmuir-Hinshelwood-Hougen-Watson rate expression.